This invention relates to a method for controlling an exposure time in consideration of the reciprocity law failure of a photosensitive material, for use in a picture reproducing machine such as a process camera, and the like.
In a conventional automatic process camera, distances between the picture holder and lens holder and between the photosensitive film holder and the lens holder are automatically determined in dependence on the reproduction scale, and then the proper exposure time for photographing conditions is automatically determined depending on the reproduction scale, the diaphragm opening size, and a density of the essential part of an original picture, in order to photograph the original picture properly and automatically.
However, the thus obtained exposure time is proper only as long as the reciprocity law of the photosensitive film used is valid, but under the photographing conditions in which the reciprocity law of the photosensitive film is not valid, the exposure time is practically improper.
In order to correct the reciprocity law failure and to obtain a proper exposure time, the photosensitive characteristics of the photosensitive material should be known, and the experience of the operator must be relied upon. Hence, under the photographing conditions of reciprocity law failure, in practice, the exposure time is automatically obtained as if the reciprocity law were valid, and then the correction time considered by the operator, depending on his experience, is added to or subtracted from the obtained exposure time. In this embodiment, therefore, in fact, the automatic exposure control is imperfectly performed.
In this case, the reason that the experimental knowledge of the operator is required for correcting the reciprocity law failure, is as follows. The photosensitive characteristics depend on the photosensitive materials, and the exposure time varies depending on the distances between the picture holder and the lens holder and between the photosensitive film holder and the lens holder at a certain reproduction scale, the diaphragm opening size, and the density of the original picture all of which synergistically affect one another. Further, it is experimentally confirmed that the exposure time is varied when the lens is exchanged so as to change the focal distance in order to vary the reproduction scale. Accordingly, since these factors synergistically affect one another, the correction time cannot be obtained according to the convention period.